There is no forensic evidence convicted serial killer Kathleen Folbigg's four young children were smothered, according to lawyers who are petitioning for a judicial review of her case.

Folbigg was found guilty of killing her two sons and two daughters over a ten-year period in the NSW Hunter Valley in 2003 and was jailed for 40 years.

A NSW Supreme Court jury found Folbigg guilty of the manslaughter of her first baby, 20-day-old Caleb, and the murders of eight-month-old Patrick, 10-month-old Sarah and 18-month-old Laura.

She was also convicted of the grievous bodily harm of her second child Patrick when he was 4 months old — over a smothering attack that left him blind.

The court heard Folbigg was unable to cope with the demands of motherhood and killed her children in fits of rage.

But in the petition made to NSW Governor-General David Hurley, Monash University Professor of Forensic Pathology Stephen Cordner wrote: "If the convictions in this case are to stand, I want to clearly state there is no pathological or medical basis for concluding homicide.

"It seems not to have been explicitly stated in the trial, but there is no forensic pathology evidence, no signs in or on the bodies to positively suggest that the Folbigg children were smothered or killed by any means."

During the trial, the prosecution relied heavily on Folbigg's diary entries about her children.

"Scared she'll leave me now, like Sarah did," she wrote of Laura in one entry.

"I knew I was short tempered and cruel sometimes to her and she left — with a bit of help."